130 CARBONICOLA, ANTHRACOMYA, AND NAIADITES. 



r Mtaltna triangularis, Barrois. Recherches sur les terrains anciens des 



Asturies et de In Gallice, p. 330, pi. xvi, 

 fig. 14, 1882. 

 Anthracopteha triangularis, Hind. Quart. Journ. Geol. Soc, vol. lxix, p. 253, 



pi. vii, figs. 7—9 a, 1893. 



Specific Characters. — Shell mytiliform, compressed obliquely, obovate with a 

 pointed anterior end, and the posterior wall, much flattened and produced obliquely 

 downwards and backwards, comprising the greater part of the shell. The 

 posterior border is regularly curved almost in a semicircle, and it passes imper- 

 ceptibly into the upper and lower borders. The hinge-line is straight, equal in 

 length to about one-third of the greatest length of the shell. At the posterior end 

 of the hinge-line the superior border of the shell slopes downwards and backwards, 

 passing gradually into the posterior border with a gentle curve. The inferior 

 border is very oblique and nearly straight. The byssal notch is situated at the 

 junction of the anterior and middle thirds. The umbones are not so tumid as in the 

 preceding species, but similar in shape and direction. An oblique ridge passes 

 obliquely downwards and backwards, but soon becomes lost on the surface of the 

 shell. 



Interior and external markings as in Naiadites modiolaris. 



Dimensions. — Fig. 31, PI. XVII, measures — 



Greatest antero-posterior . . .38 mm. 



Greatest dorso-ventral (close to posterior end) . 21 mm. 



Thickness . . . . .10 mm. 



Localities. — England: — Roof of the Hard-mine Coal, North Staffordshire. 

 Grains Ironstone, South Staffordshire. Crawstone, Coalbrookdale. Blue Vein, 

 Ebbw Vale. Scotland :— Shotts. 



Observations. — This shell only differs from Naiadites modiolaris in shape, and 

 is distinguished by its flatter form and the shape of the posterior end; a greater 

 amount of growth having taken place towards the posterior inferior extremity, 

 with the result of making the hinge-line much shorter than the extreme length of 

 (lie shell. It is very doubtful whether this condition should be regarded as of 

 specific value, but I have retained the name for the present because it is quite 

 easy to distinguish between the two forms, and typical young examples occur 

 along with the adults. At the same time it is true that many specimens occur 

 which it is difficult to determine, and a series might easily be figured connecting 

 I lie forms A', modiolaris and N. triangularis on the one hand, and N. triangularis 

 and A. carinata on the other. S o wer by 's original figure was from an imperfect 

 specimen, which 1 am unable to trace. This was incomplete at the posterior 

 superior angle, and a, hypothetical outline was dotted in. This specimen must 

 have been juvenile, for its extreme antero-posterior measurement is only 18 mm., 



